Conventionally, in a circuit of sinusoidal-wave symmetrical three-phase alternating current (AC), the electric potential of the neutral point of a Y-connection is always constant when loop circuits for respective phases are equal to each other in terms of their configuration conditions. No electric potential difference occurs between the neutral point of a Y-connection disposed on the power supply side and the neutral point of a Y-connection disposed on the load side. In this case, the applied sinusoidal wave from a three-phase power supply is an undistorted sinusoidal wave without harmonic components.
Moreover, in the symmetrical three-phase AC circuit, imbalanced conditions for the configuration of each of the loop circuits have been known to exhibit the following properties. That is, the neutral point of the Y-connection disposed on the load side is not at zero potential, but at a certain potential. These properties described above are disclosed in Patent Literature 1, for example.
Note that observation of shaft voltages requires some ingenious techniques as follows, for example. That is, the observation of the shaft voltage is attained by measuring a voltage difference between pseudo-neutral points. As pseudo-neutral points, there are designated points which include: appropriate points in the symmetrical three-phase AC circuit, and the midpoint of a voltage divider circuit that is set for convenience of the measurement.
In a practical-use symmetrical three-phase AC circuit, its three-phase power supply is sometimes imbalanced due to various factors. Alternatively, in a symmetrical three-phase AC power supply, a sinusoidal wave supplied from the three-phase power supply sometimes contains some harmonic components. In addition, some electric potentials are observed at the neutral point of the Y-connection disposed on the power supply side and at the neutral point of the Y-connection disposed on the load side. Furthermore, resulting from a variation in potential of the neutral points, a voltage is induced at the rotary shaft installed in a generator or at the rotary shaft installed in a motor. The thus-induced voltage is observed as a so-called shaft voltage. Such a shaft voltage may sometimes be applied also to a bearing inner ring which rotatably supports the rotary shaft.
On the other hand, a bearing outer ring is electrically connected to the housing or grounding point of the generator or motor. Accordingly, the bearing outer ring is different in electric potential from the bearing inner ring. That is, a potential difference occurs between the bearing inner ring and the bearing outer ring. Consequently, electrical connection of the inner ring to the outer ring, via a rolling element of the bearing, results in discharges between the outer ring, the rolling element, and the inner ring. Such a discharge leaves a discharge mark at the place where the discharge occurred. The discharge mark is called electrolytic corrosion. The occurrence of the discharge mark, i.e. electrolytic corrosion, causes problems when the bearing rotates.
For example, as disclosed in Patent Literatures 2 to 4, the generator of a three-phase power supply sometimes has an imbalanced magnetic circuit that results from errors and misalignment caused when the generator has been assembled. Such an imbalance in the magnetic circuit hinders the configuration of the symmetrical three-phase AC circuit, resulting in imbalanced three phase AC. The imbalanced three phase AC, in turn, generates an electric potential at the neutral point, which results in a shaft voltage.
Moreover, an exciting winding of the generator is supplied with power from an exciting power supply. Such an exciting power supply may be an exciting arrangement that uses a thyristor and the like. In this case, the exciting winding is applied with a voltage of a non-sinusoidal wave which contains a large amount of harmonics. The generator has an equivalent impedance component which is resulted from generator's configuration members including the exciting winding. Via such an equivalent impedance component, the applied voltage of the non-sinusoidal wave generates a shaft voltage which is attributed to the exciting power supplied from the exciting arrangement.
Moreover, as disclosed in the Patent Literatures described above, an electric generator shows the following typical phenomenon. That is, water vapors collide with the blades of a steam turbine of the generator. Upon collision, some of the water vapors are ionized and charged. The electric charge generated by the ionized vapors is conveyed to the rotary shaft via the equivalent impedance component. The thus-conveyed charge causes a shaft voltage of the generator. Such a shaft voltage of the generator has been known to cause electrolytic corrosion of the bearings.
On the other hand, Patent Literature 5 discloses the followings. That is, the motor disposed on the load side in a symmetrical three-phase AC circuit shows a shaft voltage that is caused by imbalance in a three-phase AC. The thus-caused shaft voltage, in turn, causes electrolytic corrosion of the bearings of the motor disposed on the load side. Moreover, Patent Literature 5 discloses the case in which a motor is driven through use of an inverter device. That is, in such a motor, a momentary imbalance in voltage is caused every time the power switches. In other words, the motor is such that the voltage of the neutral point varies at a very high frequency. The frequency sometimes reaches a few MHz. Thus, such variations in the voltage of the neutral point cause a shaft voltage, resulting in flowing of a shaft current. As a result, the bearings of the motor suffer from electrolytic corrosion.
In the area of electric motors, remarkable proliferation of drive technologies using inverter devices has recently been achieved. Such drive technologies using inverter devices are essentially different from the drive technologies using power supplies that can generate power of undistorted sinusoidal waves. That is, each of the drive technologies using inverter devices configures a rectangular-wave voltage source to perform a pseudo-three phase drive. Patent Literature 5 discloses the followings. That is, with the drive technology using an inverter device, a shaft voltage is generated due to variations in the electric potential at its neutral point. The generation of the shaft voltage results in flowing of a shaft current in the motor. Consequently, the bearings of the motor are prone to suffer the electrolytic corrosion.
As is commonly known, the drive technology using an inverter device is not the technology of driving on the basis of symmetrical three-phase AC supplied from a power supply of an undistorted sinusoidal wave. Thus, this drive technology cannot offer any phenomenon in which the voltages of phases cancel each other to be zero all the time That is, at the neutral point of a Y-connection of the motor, a voltage of a certain value is generated.
For example, Non-Patent Literature 1 to be described later discloses the followings. That is, the motor driven by an inverter device has a neutral point of a Y-connection. At the neutral point of the Y-connection, there occurs a periodic variation in voltage with large amplitudes associated with convex waves and square waves. The maximum voltages of such convex waves and square waves may rise to reach the power supply voltage of the inverter device.
Non-Patent Literature 1 is the article that appeared in Fuji Electric Journal Vol. 72, No. 2, p. 144-149, February 1999, entitled “Shaft Voltage of PWM Inverter-Driven Induction Motors.”
As described in Non-Patent Literature 1, the variation in voltage at the neutral point is observed as the shaft voltage of the rotary shaft.
As disclosed in such as Non-Patent Literature 1, the drive voltage of each phase supplied from the inverter device is transmitted, as an electric energy, to the outside of a stator via the following path. That is, the drive voltage of each phase is transmitted from a stator winding of the stator of the motor, through impedance components of members that configure the stator, to the outside of the stator.
The electric energy is transmitted to a rotor of the motor via distributed capacitance between the stator and rotor of the motor. Moreover, the electric energy arrives at the rotary shaft via impedance components of members that configure the rotor. The rotary shaft is positioned at the neutral point of the Y-connection which is equivalent to the symmetrical three-phase AC circuit that includes an equivalent circuit of the motor. Therefore, at the rotary shaft of the motor, some variation in electric potential is observed. This electric potential is called the shaft voltage as described above.
In the symmetrical three-phase AC circuit, there exists a third-order harmonic component for each phase. Such third-order harmonic components do not cancel each other. The third-order harmonic components are known to be observed at the neutral point of the Y-connection. In addition, an imbalanced component as well for each phase is known to be observed at the neutral point of the Y-connection.
By the way, the drive method of the motor commonly adopts inverter driving by a Pulse Width Modulation method (referred as “PWM method,” hereinafter). In cases of the inverter driving by the PWM method, the electric potential of the neutral point of the winding is not equal to zero. The neutral point of the winding shows some potential as described above.
The occurrence of variations in electric potential at the neutral point of the winding described above is analyzed from the following point of view. That is, the motor includes constituent elements that configure it. An equivalent circuit is derived from the constituent elements configuring the motor. The equivalent circuit is derived on the basis of equivalent electrical impedance components which the constituent elements have. The thus-derived equivalent circuit is treated as a symmetrical three-phase AC circuit.
In the equivalent circuit, the rotary shaft of the motor can be considered as the neutral point of the Y-connection. Therefore, it is clear that some variations in electric potential can occur at the rotary shaft of the motor.
For example, the electrically equivalent impedance components are extracted from the constituent elements that configure the motor, with each of the elements having the corresponding component. On the basis of the thus-extracted equivalent impedance components, the equivalent circuit of the motor is derived, with the circuit including the constituent elements that configure the motor. Using the thus-derived equivalent circuit of the motor, it has been attempted to calculate the shaft voltage.
Patent Literature 5, Non-Patent Literature 1, and the like disclose the followings. That is, from each of constituent elements that configure a motor, an equivalent electrical impedance component of the element is extracted. An equivalent circuit of the motor is derived by using the thus-extracted equivalent impedance components. By using the thus-derived equivalent circuit, the process by which a shaft voltage is generated is analyzed. On the basis of the result of the analysis, the occurrence of electrolytic corrosion is reduced.
The equivalent circuit of the motor can be analyzed by any of various methods that include: an analysis based on a concept of a distributed constant circuit; an analysis based on a modeled lumped-constant circuit that is derived by modeling the distributed constant circuit, with each of principal circuit elements of the distributed constant circuit being translated into a lumped constant element; and analyses by other various techniques.
Note that the motor's equivalent circuit, which includes the constituent elements that configure the motor, is different depending on the type and structure of the motor. Specifically, the equivalent circuit of a motor in which the stator winding and stator iron core are covered with an insulating resin is clearly different from that of a motor in which the stator winding and stator iron core are covered with a metal case.
Moreover, the motor's equivalent circuit is different also depending on elements that configure the rotor of the motor. For example, the equivalent circuit is different depending on whether or not the rotor includes a rotor iron core that configures a backyoke of the rotor. Alternatively, the equivalent circuit differs depending on whether the resistance of magnets is high or low, with the magnets forming magnetic poles of the rotor. In addition, it is clear that the equivalent circuit of the motor differs depending also on combinations of these factors.
As described above, the motors' equivalent circuits which include constituent elements that configure the motors are different in different types and configurations of the motors. Consequently, technologies which are considered to be optimum for reducing the electrolytic corrosion are different in different types and configurations of the motors. In other words, it is very difficult to present such technologies for reducing the electrolytic corrosion which would be commonly applicable to all types of the motors.
Thus, in general, the technology for reducing electrolytic corrosion has to be examined for each type and configuration of the motors, on a one-for-each basis. To date, a wide range of the technologies for reducing electrolytic corrosion have been proposed.
The inverter driving causes the shaft voltage, which in turn produces an electric potential difference between the bearing outer ring and the bearing inner ring. The shaft voltage contains harmonic components attributed to inverter switching. In the inside of the bearing, an oil film is disposed. If the potential difference rises equal to the voltage at which an electrical breakdown of the oil film can occur, a high-frequency electric current flows through the inside of the bearing. The flowing of the high-frequency electric current causes electrolytic corrosion of the inside of the bearing. Development of the electrolytic corrosion causes an undulately-wearing phenomenon in the inside of either the inner ring of the bearing or the outer ring of the bearing, which can cause unusual noise. With the motors, such electrolytic corrosion is a typical and problematic phenomenon to be solved.
As described above, the electrolytic corrosion is the phenomenon in which members configuring the bearing are subjected to damage caused by arc discharges. The shaft voltage causes an electric potential difference between the bearing inner ring and the bearing outer ring. The discharge current caused by the shaft voltage is such that the shaft current flows through the path, i.e. from the bearing inner ring through balls serving as rolling elements to the bearing outer ring. Thus, the following countermeasures have been proposed to reduce the occurrence of the electrolytic corrosion.
(1) Producing a conducting state between the bearing inner ring and the bearing outer ring.
(2) Producing an insulating state between the bearing inner ring and the bearing outer ring.
(3) Reducing the shaft voltage.
A specific way of implementing countermeasure (1) described above may include substitution of electric-conductive lubricant oil for the lubricant used in the bearing. However, the substitution has a problem in which the conductive lubricant oil will decrease, with time, in the electric conductivity and/or in the reliability of sliding movement, for example.
Another specific way of implementing countermeasure (1) described above may also be considered in which a brush is set to make the rotary shaft conductive. However, the brush has a problem in which wear particles come from the brush and the brush requires an additional space for the setting thereof, for example.
Yet another specific way of implementing countermeasure (1) described above may also be considered in which a slide bearing is used for the bearing. In this method, the slide bearing may be an oil-retaining bearing which is fabricated by sintering a metal and impregnating it with oil. The use of the oil-retaining bearing for the slide bearing allows the bearing to be in a conductive state.
In recent years, for motors controlled by inverter driving by the PWM method, a configuration has been in a mainstream, in which ball bearings are used at both sides of their rotor. Prior to these motors, however, a common configuration of motors had been such that slide bearings were used at both sides of their rotor.
For example, Patent Literatures 6 and 7 disclose the followings. That is, there is no occurrence of discharge in the bearings because they are conductive, resulting in no electrolytic corrosion.
Unfortunately, the use of the slide bearings is less in rotary accuracy of the rotating shaft than the use of the ball bearings. Moreover, the use of the slide bearings exhibits a large loss attributed to the bearings. Therefore, the motor using the slide bearings has reduced efficiency compared with the motor using the ball bearings.
A specific way of implementing countermeasure (2) described above may include changing of the material of the rolling elements located at the inside of the bearing, from the conductive metal such as iron to an electric insulator such as a ceramic material. This way can feature a very high effect of reducing the occurrence of electrolytic corrosion. Unfortunately, the way faces an economic problem of requiring higher costs.
Another specific way of implementing countermeasure (2) described above may also be considered to include use of insulating slide bearings for the bearings. For example, Patent Literature 8 discloses the followings. That is, the bearings employ slide bearings which are formed of a resin and have insulation properties. With this configuration, the bearing units can be made in an insulating state, resulting in no occurrence of electrolytic corrosion of the bearing units that have insulation properties.
Unfortunately, in the same manner as that described above, the rotational accuracy of the rotating shaft is lower with the slide bearings than with the ball bearings. Moreover, the sliding bearing formed of resin shows a large loss in the bearing. Accordingly, the motor using the sliding bearings formed of resin shows a lower efficiency than the motor using the ball bearings.
As a specific way of implementing countermeasure (3) described above, the method as described in Patent Literature 9 has been known. That is, a dielectric layer is disposed in the rotor to reduce the shaft voltage, thereby reducing the occurrence of electrolytic corrosion.
As another specific way of implementing countermeasure (3) described above, the method as described in Patent Literature 10 has been known. That is, a stator iron core and a conductive metal bracket are electrically short-circuited. Such a short-circuit changes electrostatic capacity between the stator iron core and the bracket, thereby reducing the shaft voltage.
Moreover, as yet another specific way of implementing countermeasure (3) described above, the method as described in Patent Literature 11 has been known. That is, the stator iron core and the like of a motor are electrically connected to a ground, i.e. the earth.